1. Field of the Invention
The present invention relates to an improved apparatus for converting plural input signals into an output signal. More particularly, it relates to an operational amplifier having an internally generated bias for stabilizing a current source, and a precision voltage reference generator using said op-amp.
2. Description of the Related Art
Operational amplifiers, or op-amps, are used to amplify a difference between two input signals, in order to generate an output signal. Typically, an op-amp consists of an input stage which converts the input signals into an intermediate signal whose current varies as the voltage difference between the two inputs changes. The varying-current intermediate signal is then converted into a varying-voltage signal via a conversion stage. An output driver generates the output signal as a function of the converted signal.
The industry standard "741" op-amp is typical of the prior art. See, Gray and Meyer, Analysis and Design of Analog Integrated Circuits. 2d ed., John Wiley & Sons, 1984, at p. 364.
The input stage typically includes a differential pair of input transistors having bases connected to receive the two input signals. The input transistors, collectors are connected to respective legs of a current mirror. One of the input transistors also supplies an "intermediate" signal, as a current, reflecting the voltage difference between the input signals.
The current mirror includes a matched pair of transistors, whose base leads are tied together and to the emitter of a mirror buffer transistor. The collector current which is drawn by the mirror buffer transistor is wasted, as it just runs to the supply terminal.
A conversion stage converts the intermediate signal from a current to a voltage. In one typical embodiment, the conversion stage consists of a transistor having its base connected to receive the "intermediate" signal and its collector coupled to a constant current source, and to the base of a buffer transistor. The emitter of the buffer transistor is coupled to generate the converted signal in response to the intermediate signal and a current signal received from the current source. The current source is stabilized by an external bias signal or other special biasing circuit associated with the current source.
Op-amps are used in "band-gap" voltage reference generators such as that shown in A. Brokaw, "A Simple Three-Terminal IC Bandgap Reference", IEEE Journal of Solid-State Circuits, Vol. SC-9, No. 6, December 1974, at p. 388.
It is desirable to have a more efficient op-amp which minimizes current consumption and heat generation, and requires a minimum number of external bias signals. It is also desirable to have a voltage reference generator, which requires no external bias signals.